staticdata: fix always-true `jl_is_datatype(t)` check in serialization (#61515)
Seems to go back to https://github.com/JuliaLang/julia/commit/cbfdb3facd
Caught by Claude:
In `jl_queue_for_serialization_`, `t = jl_typeof(v)` so
`jl_is_datatype(t)` is always true (every value's type is a DataType).
The intent is to check whether `v` itself is a DataType needing
uniquing, to force postorder traversal for the super chain. Fix by
testing `jl_is_datatype(v)` instead.
Co-authored-by: Claude <claude@users.noreply.github.com>